Touchscreens or touch panels provide an interface for displaying output and receiving input, a structure that makes them well suited for mobile phones, personal digital assistants, digital music players, and other compact devices. Using a touchscreen, a user can select or manipulate items displayed on the touchscreen, such as buttons, sliders, scroll wheels, and other screen icons.
Prior art systems use different methods to detect the presence of an object, such as capacitive charge transfer methods and relaxation OSC methods. In a capacitive touchscreen, for example, charge is transferred to sensing elements and objects adjacent to them. The combined stored charge is then read, with charges above a threshold indicating that the object is above the sensing element currently being read. By arranging sensing elements into a grid pattern having rows and columns, the particular location of an object on the grid can be determined.
FIG. 1 is a block diagram of prior art capacitive touchscreen 100. The touchscreen 100 comprises a surface containing multiple sensing elements 101, each configured to sense the presence an object, such as a finger 105, adjacent to it. By detecting which ones of the sensing elements the object 105 is adjacent to, the location of the object along the surface of the touchscreen 100 is determined. The capacitive touchscreen 100 functions by opening and closing switches A, A′ B, B′, C, and C′ to transfer charge that is measured by measuring circuits 110 and 115 and calculated by calculation means 120. Those skilled in the art will recognize that the capacitive touchscreen 100 requires multiple clock phases and multiple charge transfer cycles to determine whether an object is adjacent to its surface.
The touchscreen 100 has several disadvantages. For example, charge must be transferred to external capacitors CS1 and CS2 several times to measure capacitance change. This increases the latency and thus reduces the maximum frame rate that can be supported. As a result, the touchscreen 100 has limited sensitivity and a reduced signal-to-noise ratio.
The touchscreen 100, like other prior art capacitive touchscreens, also suffers from “ghosting,” during which the actual locations of simultaneous touches cannot be resolved. When simultaneous touches occur at multiple locations, the system is only able to determine that several touches occurred, the “real” touches and “ghost” touches. The system is unable to easily distinguish between the real and ghost touches. This ambiguity, and the processing needed to resolve it, increases exponentially with the number of simultaneous touches.
Some capacitive touchscreens determine simultaneous touches using “mutual capacitance,” a process that senses capacitances at the intersections of row and column lines. Systems that support mutual capacitance measurements require much more complex analog hardware, which results in higher power dissipation, lower throughput rate, larger die size, and more complex signal processing.